Photovoltaic solar cells are used to convert sunlight energy into electricity by absorbing, using semiconductor materials, photons having an energy larger than the semiconductor bandgap, the absorption of photons causing the generation of photocarriers (electrons and holes).
The electrons and holes are separated across a p/n junction that has been formed by doping adjacent regions of the semiconductor. Single junction solar cells have one p/n junction. It is well-known that the solar energy conversion efficiency of single-junction solar cells is limited due to the fact that the Sun emits photons in a broad range of wavelengths/energies, whereas the one p/n junction of a single junction solar cell has only one fixed bandgap energy. The solar photons with an energy equal or slightly greater of the bandgap of the semiconductor are absorbed and converted into electricity efficiently. However, photons with excessive energy compared to the bandgap of the single junction solar cell waste their excess energy and, photons with less energy than the bandgap of the single junction solar cell are not absorbed.
It is also known that higher conversion efficiencies can be obtained with photovoltaic solar cell devices that have multiple p/n junctions electrically connected in series. Such devices are typically referred to as multijunction solar cells. A multijunction solar cell can have any number of p/n junctions; typically, it will have has 2, 3, or 4 p/n junctions, which can be referred to as subcells. In addition to the p/n junction itself, a high-efficiency subcell will typically incorporate other functional elements such as a window layer and a back surface field layer.
As a particular example, a three-junction solar cell (also referred to as a solar cell device or, simply as a device) has three subcells, which can be referred to as a top cell (TC), a middle-cell (MC), and a bottom cell (BC). The three subcells are typically electrically connected to each other in series using tunnel-junctions, metal layers, or other equivalent components. Each subcell has a respective bandgap, which is chosen in accordance with the solar spectrum of interest and with the goal of optimizing the conversion efficiency of the three-junction solar cell (e.g., see U.S. Pat. No. 7,863,516 incorporated herein by reference in its entirety).
The subcells can be grown monolithically by epitaxy or assembled mechanically. In either case, the subcells generating the least photocurrent will be limiting the overall current that the multijunction solar cell can generate because the subcells are connected in series. As such, when measuring the performance metrics (e.g., the electrical, optical, electro-optical characteristics) of multijunction solar cells, the individual performance metrics of the constituent subcells are often concealed and/or not easily quantifiable with currently available test apparatus.
As will be understood by the skilled worker, the measurement and characterization of all the performance metrics of each individual subcell is desirable and can be important in order to better optimize the performance of multijunction solar cells.
In the prior art, a technique exists for determining the spectral response of a multijunction solar cell. The technique is typically referred to as the quantum efficiency (QE) measurement, also known as internal quantum efficiency (IQE) or external quantum efficiency (EQE) depending on whether or not the light reflected from the device or other shadowing effects are taken into account. The QE measurement for a single junction device is straightforward and the prior art technique is typically adequate for single junction cells. In that case, monochromatic light illuminates the solar cell and the solar cell's response is measured for the spectral range of interest. However, for the QE measurement of multiple-junction cells, the monochromatic light is absorbed by only one of the subcells while the other subcells generate no photocurrent due to the mismatch between the probe photon energy and the absorption characteristics of other subcells.
This results in no overall photocurrent due to the series connection of the subcells, and it is therefore not possible to measure the QE of the individual subcells by simply scanning the wavelength of the probe light in such a multijunction configuration. Instead, in order for the QE of the different subcells of a multijunction device to be measured, a light bias and/or a voltage bias needs to be applied to the other subcells that are not being probed optically (see for example Woodyard et al, Proceedings 25th PVSC, May 13-17, 203-206, 1996). However, the choice of the intensity of light used to bias the other subcells in the QE measurement, or the choice of the voltage bias used simultaneously can affect the results unpredictably. Subjective choices of light and voltage bias settings during the QE measurement of multijunction cells can lead to erroneous results which can negatively impact the design cycle and deployment time of the multijunction solar cells.
Other techniques have also been developed in the past for multijunction cells to try to account for the spectral mismatch corrections of Sun simulators. An example of such a technique is described in Adelhelm et al. (Solar En. Mat. and Sol. Cells, 1998; 50: 185-195). Occasionally, such techniques involve mathematical procedures to evaluate the spectral mismatch correction, as described for example in Meusel et al. (Prog. Photovolt: Res. and Appl, 2002; 10:243-255). Typically, these techniques attempt to characterize the devices of interest with a Sun simulator, which attempts to match the overall Sun spectrum for spectral conditions typical close to AM1.5 direct, AM1.5 Global and/or AM0. Or the Sun simulator is adjusted to compensate for spectral corrections for the different subcells, such that each subcell generates the photocurrent that would be expected under the reference spectra of interest.
An effective irradiance can also be defined to characterize the spectra fidelity of the Sun simulator and the effective irradiance can be normalized or compared to the effective irradiance of the reference spectrum of interest to define measurement conditions that can help the interpretation of the performance data measured under such conditions. However, trial attempt measurements and multiple adjustments of the light and bias settings during the QE measurement, or other similar optical characterization techniques attempting to mimic spectral conditions close to a reference spectrum in the assessment of the performance of multijunction cells are time consuming and can still lead to erroneous results given that several combinations of light and voltage biases are possible.
As shown in the prior art above, it is particularly complicated because of the fact that precisely reproducing a reference spectrum is difficult and might still require additional characterization and mathematical procedures to evaluate the spectral mismatch correction. The corrections and adjustments are typically attempted without the full characterization of each constituent subcells of the multijunction cell, measured under well controlled illumination conditions, and without sweeping large enough ranges of illumination intensities in the subcells of interest. This is especially of concern in the cases of particular interest when the properties of some of the subcells are non-ideal due to material and/or fabrication issues.
Therefore, an apparatus capable of properly characterizing the individual performance metrics of the constituent subcells of multijunction photovoltaic solar cells is desirable.